In a typical cellular network, also referred to as a wireless communication system, a user equipment communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs). The network may be for example a Third Generation (3G) network based on e.g. High-Speed Packet Access (HSPA) in Wideband Code Division Multiple Access (WCDMA). HSPA refers to both improvements, in relation to Universal Mobile Telecommunications System (UMTS), made in the Down Link (DL), often referred to as High Speed Downlink Packet Access (HSDPA) and to improvements made in the uplink, often referred to as High Speed Uplink Packet Access (HSUPA). Today's release of the WCDMA standard (2×2 MIMO and 64 QAM) is a 3G technique which uses a carrier bandwidth of 5 MHz per carrier and a data transfer rate up to 42 Mbps per carrier. WCDMA used Code Division Multiple Access (CDMA). WCDMA supports Frequency Division Duplex (FDD). A FDD carrier comprises 5 MHz frequency for downlink and another 5 MHZ frequency for uplink (separation is normally 60 MHz).
A User Equipment (UE) is a device which may access services offered by an operator's core network and services outside operator's network to which the operator's RAN and CN provide access, e.g. the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop, or PC. The user equipment may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly with the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and one or possibly more core networks and possibly the internet.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called NodeB (NB), or base station. A cell is a geographical area where radio coverage is provided by the base station at a base station site. The base stations communicate using Radio Links (RL) with the user equipments within coverage range of the base stations.
In the DownLink (DL), i.e. in the direction from the base station to the user equipment, there are physical radio channels per cell and in the UpLink (UL), i.e. in the direction from the user equipment to the base station, there are physical radio channels per user equipment. Physical channels allocated to a particular user equipment are called dedicated. There are also common and shared physical radio channels. A radio link (to/from a user equipment) is made up of one or more dedicated physical channel. HSDPA (DL) is using shared channels (time division). HSUPA (UL) is using dedicated channels.
Mobility refers to that the user equipment is able to moves between cells, radio access technologies etc. while still keeping its connection. The user equipment may experience different degrees of mobility. The term handover refers to the transfer of a user equipment's connection from one radio link to another, where each radio link is in a separate cell. In the WCDMA standard there are two main categories of handover of a user equipment: hard and soft. From the access network point of view there is also a variant of soft handover, called a softer handover. Hard handover is a category of handover procedures where there is only one radio link (one cell) at any one time and mobility handover from one to the other is done at a synchronized point in time. Soft handover means there are two or more radio links at the same time, so that newer radio links are established before older are abandoned. Soft handover is a category of handover procedures where the radio links are added and abandoned in such manner that the user equipment always keeps at least one radio link to the UTRAN. Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same base station, when one base station serves several cells like in a typical sectorized site configuration (3 sectors in 120 degree angles). The base station can then combine radio links from several cells before decoding the transferred information.
In 3G/HSPA/WCDMA a user equipment may be connected to multiple base stations via multiple radio links at the same time, to support mobility for the user equipment without interruption. As mentioned above, this is called Soft Handover (Soft HO). The set of radio links belonging to one base station involved in softer handover to a user equipment is called a Radio Link Set (RLS). According to 3GPP, a RLS is a “set of one or more Radio Links that has a common generation of Transmit Power Control (UL-TPC) commands in the DL” (vice versa, also valid for DL-TPC in the UL). Soft handover for a user equipment is made between RLSs, one RLS per base station. A RLS belongs to a base station. TPC is an abbreviation for Transmit Power Control or Transmit Power Commands and is a mechanism used in order to prevent too much unwanted interference between network cells and user radio links in a WCDMA RAN. Cell power is a shared resource in WCDMA and abundant power is undesired. In general for any radio access network power control enables reduce energy consumption. The UL-TPC mechanism dynamically adjusts the UL transmission power. For each radio link uplink, the uplink inner-loop power control continuously adjusts the user equipment transmit power in order to keep the received uplink SIR at a given SIR target. Each radio link comprises an UL and a DL. The UL carries TPC commands to control DL power (this is called DL-TPC in this document). The DL carries TPC commands to control UL power (this is called UL-TPC in this document). UL-TPC commands from different RLS may and will differ since they are separated physically and logically separated (so far).
Efficient power control is crucial in CDMA technology communications network in order to minimize interference between radio channels in the network. Power control ensures that each user equipment receives and transmits just enough energy to properly convey information while interfering with other user equipment's no more than necessary.
Closed loop power control comprises two loops: inner loop and outer loop. The inner loop power control in the uplink refers to the ability of the user equipment to adjust its output power in accordance with one or more TPC commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target. The inner loop power control is also in the downlink and refers to the ability of the base station to adjust its output power in accordance with one or more TPC commandos received in the uplink. The user equipment generates DL-TPC commands to control the network downlink transmit power and send them in the TPC field of the uplink DPCCH. Upon receiving the TPC commands, the UTRAN adjusts its downlink DPCCH/DPDCH power accordingly. In the corresponding manner, the base station generates UL-TPC commands to control the user equipment transmit power. Upon receiving the UL-TPC commands, the user equipment adjusts its uplink power accordingly. Outer loop power control is used to maintain the quality of communication at the level of bearer service quality requirement, while using as low power as possible. The uplink outer loop power control is responsible for updating a target SIR in the base station for each individual uplink inner loop power control. This target SIR is updated for each user equipment according to the measured uplink quality, e.g. UL BLER BLock Error Ratio (other example: residual BLER alternatively retransmission rate in case of HSUPA HARQ—Hybrid Automatic Repeat Request) for each Radio Connection (RC). A radio connection is an aggregation of all radio links for one user equipment). The downlink outer loop power control is the ability of the user equipment receiver to converge to required link quality (DL BLER) set by the network in downlink. Outer loop Power control may be performed periodically or occasion (if no data block transmitted for a while, nothing to probe on), but obviously with a lower update rate than the inner loop (1500 Hz), for example every 2-3 ms (rather 2-200 ms).
A separate uplink power control loop is made by each base station, i.e. per RLS for the user equipment, to keep uplink interference low in the cell. In other words, there is one UL-TPC command stream per RLS for the user equipment, and there is a risk that these UL-TPC command streams compete against each other in case they often contradict and strive in different directions, e.g. one with a net upwards and the other with a net downwards. Each UL-TPC command stream is sent in downlink from the base station to the user equipment to control uplink power from the user equipment. In Soft HO, multiple RLS uplink power control loops are connected to same user equipment. The user equipment combines UL-TPC command streams from each RLS. The UL-TPC command streams, for a slot, may differ per RLS, due to different radio link conditions per RLS. A slot is, in WCDMA, a 10 ms radio frame which is divided into 15 slots or 0.666 ms each. The user equipment combines, e.g. by using AND, the different UL-TPC command streams in order to determine the resulting uplink power control decision. Logical AND may be used with the objective to choose the best RLS (or rather RL since user equipment cannot tell the difference between different RLS), meaning that the RL with the strongest uplink (smallest cell radio channel path loss) and this way minimize uplink interference in that cell.
There are near/far uplink/downlink imbalance problems related to the difference in cell downlink output power and this situation is becoming more common as more heterogeneous networks are deployed, e.g. mixing high power cells with low power cells. This may be referred to as Heterogeneous Network (Hetnet). The terms high power and low power cells will be described in more detail below. Generally, the UpLink Received Total Wideband Power (UL RTWP) level in a cell will experience more instability in Hetnet during Soft HO situations. Moreover it may be expected that areas selected for reinforcement with micro cells have higher density of user equipments which amplifies the imbalance problem. UL RTWP instability will result in need for larger margins for call setup and for Enhanced UL scheduling headroom HSPA, in turn leading to a lower average cell throughput in uplink. UL RTWP also affects the user equipment's call drop rate negatively.
A scenario illustrating the uplink power instability is shown in FIG. 1. In this scenario illustrated in FIG. 1, the uplink imbalance in a communications network 100 at the point where a user equipment 101 is near to exit soft HO to the right will be considerable. The imbalance is because DL and UL HO areas do not match. The imbalance increases as further away the user equipment is from the ideal UL shift HO area. FIG. 1 shows a high power cell 105a and a low power cell 105b. In FIG. 1, the high power cell 105a is synonymous with a macro cell. Furthermore, the low power cell 105b is synonymous with a micro cell (in some contexts a pico cell). A macro cell is a cell that provides radio coverage served by a high power base station. A macro cell has typically a power output in tens of watts and has a cell radius of e.g. 1-10 km. Generally, macro cells provide coverage larger than a micro cell. A micro cell is a cell that provides radio coverage served by a low power base station. A micro cell has typically a power output lower than a macro cell and a cell radius of e.g. less than 1 km. The micro cell covers a limited area such as a mall, a hotel, or a transportation hub and they are deployed in order to add network capacity in areas with very dense phone usage.
In FIG. 1, the high power cell 105a is illustrated by the base station serving the high power cell 105a and the low power cell 105b is illustrated by the base station serving the low power cell 105b. The user equipment 101 is in handover between the high power cell 105a and the low power cell 105b. The left most dotted vertical line 115 illustrates the optimal UL handover point for the user equipment 101 of equal uplink path loss. Seen from the left, the distance between the second dotted vertical line 117 and the fourth dotted vertical line 118 represents the soft handover region 120 for the user equipment 101. The third dotted vertical line 125, seen from the left, represents the optimal DL handover Point of equal down link Common Pilot CHannel (CPICH), i.e. a point for a serving cell change.
CPICH, as mentioned above, is a downlink channel broadcast by base stations with constant power and of a known bit sequence, and received by user equipments. Its power is usually between 5% and 15% of the total base station transmit power. Commonly, the CPICH power is 10% of the typical total transmit power of 43 dBm. The common pilot channel is a code channel, which is scrambled by the cell specific scrambling code. The CPICH is for aiding the channel estimation for dedicated channels and for providing the channel estimation reference for common channels. Two types of CPICH are defined, the primary and the secondary common pilot channel (P-CPICH & S-CPICH).
FIG. 2 illustrates a typical guideline of how the configuration should not look like, i.e. a wrong configuration in today's network. The power setting are balanced by setting CPICH within bounds, e.g. 5%-15% of nominal power and the difference between CPICH is set to max 4 dB. The user equipment 101 is served by a base station in a high power cell 105a and generates high interference in the adjacent cell, the low power cell 105b. The user equipment 101 is in handover between the two cells. The used handover region 201, which is downlink based, is the area where the two cells overlap. The ideal handover region 205 for the uplink, which is uplink based, is illustrated to the left of the used handover region 201.
Soft HO (Stationary, or Mobile but at Least Keeping within Soft HO Area)
A scenario comprising soft handover for a stationary, or at least not moving so much that radio links are added or deleted hence not involving UL synchronization of new radio links or RLSs, user equipment will now be described with reference to FIG. 3. In FIG. 3 and in the following description, the single underline indicates that the received downlink is ok and the double underline indicates that the received downlink is bad. An ok downlink means that it is strong and continuously decodable. A bad received downlink means that it is weak and risking times being non-decodable in a varying fading radio channel. Seen from the top, the first row in FIG. 3 illustrates the UL TPC command bit sequence for the micro cell, μRLS 301. The second row in FIG. 3 illustrates the UL TPC command bit sequence for the macro cell, mRLS 303. The third row in FIG. 3 illustrates a combination of the UL TPC command bit sequence for the micro cell μRLS 301 and for the macro cell, mRLS 303 for the user equipment, referred to as AND (UE) 305 in FIG. 3. The fourth row in FIG. 3 is a graph which illustrates the radio link uplink power, either referred as the RBS Received Signal Code Power (“UL RSCP”) or the UE transmitted power (“UE TxPwr”) 307 in FIG. 3. The UL-RSCP denotes the power received by the base station on a particular RL or RLS. Note that when the downlink is bad there is no AND since there is nothing to AND with, only the “all ones” UL-TPC stream in the remaining RLS.
In FIG. 3 the UL TPC command bit sequence for the micro cell RLS, μRLS 301, is exemplified to be 0101010101 0101010101 000010101010. A “0” mean decrease UL power and a “1” means increase UL power. The UL TPC command for the pRLS 301 between the dotted lines is associated with a bad downlink 310. The bad downlink 310 has the UL TPC command bit sequence 0101010101. The UL TPC command bit sequence for the macro cell RLS, mRLS 303, is exemplified to be 1111111111 11111111 11111111111. When combining, by adding, the UL TPC command bit sequence for the micro cell RLS with the UL TPC command bit sequence for the macro cell RLS, the result, AND (UE) 305, for the user equipment is 0101010101 11111111 000010101010. As seen from the graph 307 in the lower part of the FIG. 3, a large uplink power spike due to fading dip in the weak downlink of that same RLS. The strongest RLS in UL is the weaker RLS in DL for the micro cell and vice versa for the macro cell. In UL is all depends on the radio channel path loss while for the DL it also depends on the DL power which is always relative to the CPICH power. The micro cell RLS keeps sending UL-TPC down-commands more often than the other RLSs.
Also without complete temporary outage of micro DL, i.e. logical AND between two inputs still possible, there will be instability and power rushes as illustrated in FIG. 5 for when “2 RL” because zeros will dominate (00001000010001001000) in the micro cell RLS while in the macro cell RLS there will be mostly ones (1110110111011110). This results in unpredictable competition between the power control loops. Compared to a pure a macro cell network this will results in increased UL RTWP instability in the macro cell and especially in the micro cell. The base station received signal code power, i.e. the transmission power of the user equipment (UE TxPwr), is illustrated in the graph in the lower part of FIG. 3. It is seen that the bad downlink dips lead to a power rush in the user equipment's transmission power which creates interference and instability.
Soft HO Entry (Mobility, Involving Addition of Radio Link in New RLS)
FIG. 4 illustrates mobility of the user equipment in soft HO entry. This scenario involves time for UL synchronization of the added RLS, comparing FIG. 4 with FIG. 3. A user equipment moves out from the micro cell and enters soft handover with the macro cell, i.e. the UE moves from the coverage area of the low power node to the coverage area of the macro node, while the user equipment is still being located in the HO area. In FIG. 4, the single underline indicates an ok downlink and the double underline indicates a bad downlink. Seen from the top, the first row in FIG. 4 illustrates the UL TPC command bit sequence for the micro cell, μRLS 401. The second row in FIG. 4 illustrates the UL TPC command bit sequence for the macro cell, mRLS 403. The third row in FIG. 4 illustrates a combination of the UL TPC command bit sequence for the micro cell μRLS 401 and for the macro cell, mRLS 403 for the user equipment, referred to as AND (UE) 405 in FIG. 4. The fourth row in FIG. 4 is a graph which illustrates the radio link uplink power, either referred to as the base station Received Signal Code Power (UL RSCP) or the user equipment transmitted power, referred to as UL RSCP (UE TxPwr) 407 in FIG. 4. The UL-RSCP denotes the power received by the base station on a particular RL or RLS. After some time, uplink synchronization is achieved in the mRLS 403, denoted with UL Synch (mRLS) 410 in FIG. 4. Note that when the downlink is bad there is no AND since there is nothing to AND with, only the “all ones” UL-TPC stream in the remaining RLS.
In FIG. 4 the UL TPC command bit sequence for the μRLS 401 is exemplified to be 0101010101010101 0101010101 000010101010 1100101011. A bad downlink 413 in the μRLS 401 is associated with the UL TPC sequence 0101010101. In the start, when the user equipment is located only in the micro cell there is no UL TPC sequence associated with the macro cell, illustrated with a thick line 415 in the mRLS 403. When the user equipment enters soft HO with the macro cell (radio link addition), the UL-TPC power is illustrated with only ones in the row associated with the mRLS 403, i.e. the UL TPC command bit sequence for the mRLS 403 after Soft HO entry but before UL synch achieved in the macro cell is 1111111111 11111111 11111111111.
The UL Power rushes arises and amplifies when UL synch is delayed (not ideal due to fading). The synchronization relates to Radio Link addition in a SoftHO scenario. UL synchronization will be delayed since UL path loss to the macro cell is relatively large. Before UL synchronization is achieved typically the UL TPC sequence in the old micro RLS is ideally 010101010101, while the UL TPC sequence in the new macro RLS is all ones, 1111111111. The reason for this is the logical AND between the received sequences performed in the user equipment. When the user equipment has entered soft HO with the macro cell, it performs a combining of the UL TPC bit sequences from the micro and macro cell. In the example shown in FIG. 4, the result of the combination is 0101010101 11111111 0000101010101 010010101. In the part with the bad downlink 413 in the micro cell, the ones in the macro cells are the only commands visible to the user equipment and hence the result is all ones. Compared to a pure macro cell network (traditional deployment) this will give more and longer duration UL Received Signal Strength Indicator (RSSI) spikes in the micro cell, when fading of micro RLS occurs. The UL RSCP (UE TxPwr) 407 illustrates that there is a power rush associated with the bad downlink 413 which creates interference and instability. The soft HO entry example illustrated in FIG. 4 is when the user equipment is moving from a Low Power Node to a Macro node (RL Addition), while the user equipment is still in the HO area. The scenario in FIG. 4 may also be referred to as showing UL-TPC command streams in a non-Coordinated SoftHO scenario. The downlink is exposed to fading in this scenario.
The unbalanced soft handover problem is very serious.
The larger the difference between the cell's DL powers, the worse UL cell interference and UL user equipment link instability gets. A very bad radio performance situation will arise, a very instable situation with a fading radio channel resulting in frequent toggling between two extremes:                Very high user equipment transmission power in the micro cell giving excessive SIR and severe interference. This leads to performance degradation bot for R99 uplink and Enhanced uplink, EUL).        Very low user equipment transmission power in the macro cell giving bad SIR and large risk of losing uplink synchronization. This will probably affect the drop rate of the user equipment's call sessions negatively.        
Deploying Hetnet, meaning small output power cells inside coverage of large macro cells, is aiming to increase capacity and/or user data rates in a limited area, but instead it instead risks effectively worsening the performance and capacity sometimes if circumstances are unfortunate.
In a handover situation the user equipment EUL performance shall typically rely on the uplink of the micro cell despite it is often not the serving cell. Handover decisions are based on downlink power, which correlates badly to the optimal uplink handover region, see FIG. 2.
Concluding, if soft handover is necessary (softer handover not possible in the network deployment) the entire Hetnet idea with embedded small capacity enforcement cells is in jeopardy.
If the handover is a soft handover, the micro cell will have its own separate UL inner power control loop (in a separate non-serving RLS). Due to the imbalance, the two power control loops for the micro cell and for the macro cell will constantly compete against each other. One will be dominated with up commands while the other loop will be dominated with down commands.
In general it is best to make sure a critical handover border is a softer handover border. If a soft border may be transformed into a softer border it would make the instability troubles go away, because it implies having only one UL inner power control loop, i.e. the separate UL inner loop for the micro cell. Such transformation is however not always an option, either due to that the base stations are physically separated or the Centralized RAN hardware pools are already full with other softer cell neighbors (borders).
The FIG. 5 shows the impact on uplink power and SIR depending on the position of the user equipment. The macro cell, i.e. the high power node is assumed to be on the left and the micro cell, i.e. the low power node is assumed to be on the right. Starting from the top, the graphs in the first row represents the RRSI of the micro cell, denoted Micro RRSI 501 in FIG. 5. Two graphs illustrate one RLS, three graphs illustrates two RLS. The second row illustrates the SIR for the micro cell, denoted Micro SIR 503 in FIG. 5, where all three graphs illustrate two RLSs. The SIR target 505 is illustrated with a horizontal line in the graphs illustrating Micro SIR 503. The third row illustrates the SIR for the macro cell, denoted Macro SIR 507 in FIG. 5 with the SIR target 505 illustrated with a horizontal line. The Macro SIR 507 is illustrated with one graph for one RLS and three graphs for two RLSs. The fourth row illustrates the resulting handover region 510 which is seen to be balanced for the downlink, but tense for the uplink. The fifth row illustrates the ideal downlink handover region for the downlink, denoted Ideal DL HO region 513 in FIG. 5. The sixth row illustrates the ideal downlink handover region for the uplink, denoted Ideal UL HO region 515 in FIG. 5. Vertically, FIG. 5 illustrates different numbers of RLSs. The x-axis of all graphs in FIG. 5 represents the time and the y-axis in FIG. 5 represents the power. A thick vertical dotted line illustrates where the uplink is equal, dented equal UL 517 in FIG. 5. The equal uplink 517 is seen in FIG. 5 to be in the ideal UL HO region 514. Another thick vertical dotted line illustrates the micro border 519. One thick vertical dotted line illustrates the equal DL 521. A thin vertical line illustrates the macro border 523, which is seen to be at the same point as the forced macro UL border 525.
FIG. 5 shows an uplink power graph with power rushes in the micro cell in the HO region, due to use of legacy Soft HO in an UL/DL imbalance scenario. In the FIG. 5 it is seen that the ideal HO region for the uplink 515 and the ideal HO region for the downlink 513 only partly overlaps, and this causes the problem with unstable uplink power control.